Radiation is one of the key treatment choices for advanced prostate cancer (PCa), and radiation is an adjuvant treatment or alternative to surgery for organ-confined PCa. External beam radiation is associated with significant toxicities to the neighboring tissues. Combining radiation therapy with radiation sensitizers for PCa would permit lower doses of ionizing radiation with higher efficacy - this would both improve the therapeutic outcomes, and limit the common and debilitating side-effects of radiation. The objective of this application is to establish whether activation of transcription factor Stat5a/b determines the response of PCa to radiation therapy. We will test the following hypothesis: Active transcription factor Stat5a/b induces resistance of PCa cells to radiation therapy via promotion of double-strand DNA break repair. The rationale for this study is based on the applicants' previous findings demonstrating that: 1) Stat5a/b is critical for viability of Stat5a/b-positive PCa cell lies, for growth of human PCa xenograft tumors in nude mice and Stat5a/b promotes metastatic behavior of PCa cells in vitro and in vivo; 2) Stat5 regulation of PCa cell viability is mediated through androgen receptor (AR)-dependent and AR-independent mechanisms, 3) Elevated nuclear Stat5a/b levels in PCa predict early disease recurrence and 45% increased likelihood of PCa-specific death; 4) Nuclear Stat5a/b is over-expressed in castrate-resistant clinical PCas 5) Stat5a/b inhibition sensitizes PCa cells to radiation; 6) Inhibition of Stat5a/b increases radiatio-induced DNA- damage in PCa cells and 7) Stat5a/b up-regulates Rad51 which is the key protein in Homologous Recombination (HR)-DNA repair in mammalian cells. Specific aims: Aim 1) Determine whether Stat5a/b promotes radiation- induced a) HR double-strand (ds)DNA break repair through up-regulation of Rad51 and/or b) NHEJ DNA repair in PCa cells. 2) Determine whether inhibition of Stat5a/b sensitizes human PCa cell lines and xenograft tumors to radiation-induced regression. We will also establish the specificity of Stat5-inhibition-induced radiosensitization to PCa vs. normal surrounding tissues. 3) Establish whether elevated Stat5a/b levels in clinical human PCas predicts poor therapeutic response to radiation therapy. Innovation: The concept of Stat5a/b regulation of Rad51 and dsDNA break repair in PCa is entirely novel. Also, the concept of Stat5a/b inhibition as a strategy to sensitize human PCa to radiation is completely unexplored. The concept of active nuclear Stat5a/b as a predictive marker of PCas with poor response to radiation therapy is new. The proposed studies include unique PCa material from patients and unique research tools. Significance: The proposed studies hold the potential to identify a new strategy for sensitizing human PCa cells to radiation based on inhibition of Stat5a/b. This is important since development of a pharmacological radiation sensitizer for PCa would enable more effective destruction of the cancer cells and reduce the radiation therapy-related side-effects. The proposed work has direct significance for therapy development for PCa since existing small-molecule Jak2-Stat5 inhibitors could become immediately available to prostate cancer patients as pharmacological tools for radiation sensitization.